An internal combustion engine is an engine having a combustion chamber into which fuel and an oxidizer are directed and combusted. High pressure gases are applied to movable components of the engine, which are harnessed to do work. For instance, gasoline is mixed with air in the cylinders of an automobile engine and ignited by spark plugs to drive a crankshaft during operation of the automobile. Pressure is exerted against the piston heads within the combustion chambers.
Another type of engine is a ramjet. A ramjet has no moving parts and achieves compression of intake air by the forward speed of the aerial vehicle to which it is attached. High pressure is produced by “ramming” external air into a combustor using the forward speed of the vehicle. Air entering an intake of a supersonic aircraft is slowed by aerodynamic diffusion created at the air intake or inlet and diffuser. The speed of the air is slowed to velocities comparable to those in a turbojet engine. The expansion of hot gases after fuel injection and combustion accelerates the exhaust air to a velocity higher than that at the inlet and creates positive push. In a turbojet engine, the high pressure in the combustor is generated by a compressor. The lack of a compressor in a ramjet renders it lighter and simpler in operation than a turbojet. One drawback to a ramjet is that it only produces thrust when the engine is already moving forward relative to the surrounding ambient air.
Scramjet is an acronym for Supersonic Combustion Ramjet. The scramjet differs from the ramjet in that combustion takes place at supersonic air velocities through the engine. It is mechanically simple, but more complex aerodynamically than a jet engine. Hydrogen is normally the fuel used. Because the scramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a projectile, which must carry all of its oxygen. Scramjets are ideally suited for hypersonic flight within the atmosphere.